The mechanisms of attention are a fundamental question for cognitive neuroscience. In recent positron emission tomography (PET) study (Corbetta et al, 1991) subjects detected changes in successive stimulus arrays along three dimensions (color, form, or motion) Selective attention to a dimension activated the relevant extrastriate cortex, suggesting channel-specific gating. Additionally, orbital frontal and lateral premotor cortex were activated, suggesting a possible role in late selection processes. While well-designed, this study suffers from the limited temporal resolution of rCBF scans. Such temporal information can be provided by event-related potentials (ERPs) which provide millisecond time resolution, albeit with poor spatial resolution. This laboratory had demonstrated that rCBF data can facilitate the localization of ERP data (Heinze et al., 1994) which in turn provides information about time course. The investigators propose to replicate the study by Corbetta and his colleagues, adding the temporal resolution of ERPs to the spatial resolution of rCBF methods. Doing so will provide more information about the attentional processes, as well as verifying that the regions were in fact active during the stimulus-response period.